84 General Bacteriology. 



EXERCISE LXX. QUANTITATIVE ANALYSIS OF niLK. 



a. Obtain a sample of milk in a sterile vessel. 



b. Dilute milk 1000 times with sterile water. 



c. Make plates as under soil (LXVIII). 



d. Count colonies and estimate number of bacteria per cc. 



EXERCISE LXXI. EFFICIENCY OF PASTEURIZATION. 



a. Place same milk as used in previous experiment in the bottles of a pasteurizing 

 apparatus, such as Freeman's, and pasteurize as per printed directions, or place the milk 

 in ordinary milk bottles or fruit jars, filling 



to a uniform level; these are then to be 

 placed in a flat bottomed pail which is to be 

 filled with water and heated to 71 C.(160 F. ) , 

 remove source of heat, cover and allow to 

 stand 30 minutes. Remove bottles and cool 

 as quickly as possible without danger to glass. 



b. Determine bacterial content of pas- 

 teurized product by making plates. A dilu- 

 tion of 100 will probably be sufficient. Ex- 

 press results so as to indicate per cent, of or- 

 ganisms destroyed by the process. Compare 

 the keeping qualities of the pasteurized pro- 

 duct with that of the raw milk by keeping 

 samples of both under similar conditions, 

 e. g. in locker or ice chest, making frequent 

 observations. 



Pasteurized milk should not have a permanently cooked taste. 



REFERENCES. Bull. Wis. Exp. Station No. 44. Russell, Outlines of Dairy Bacte- 

 riology, 95 (4th Edit.). 



FIG. 17. A home-made pasteurizer (Russell.) 



EXERCISE LXXII. TESTING ANTISEPTIC ACTION OF CHEMICALS. 



GENERAL DIRECTIONS. 



o. Fill a number of test-tubes with a measured quantity of agar (5 cc.). 



b. Add to the agar varying but measured amounts of the substance to be tested. If 

 the antiseptic is not volatile, or affected by heat, sterilize. 



c. Inoculate the tubes thus prepared, together with a control, with B. coli and make 

 rolls. 



d. Keep these cultures under observation in the 28 C. incubator- 



e. If no growth appears within 96 hours repeat the experiment, using smaller amounts 

 of the antiseptic. In this way determine the amount of chemical (in %) which just pre- 

 vents growth. 



SPECIAL DIRECTIONS. Test in this way carbolic acid (5 %), alcohol (95 %). 

 REFERENCES. A. 566; H. 411; N. 527; S, 156. 



